Generally, a conductive paste is composed of a binding agent such as a liquid-form epoxy resin or a liquid-form phenol resin, and a conductive powder (metal particles) composed principally of silver powder. In a conductive paste in which an epoxy resin is used as a binding agent, a hardener such as a polyamide resin or an amine is used, for the purpose of increasing or decreasing the curing speed. In the case of such a conductive paste, when the binding agent in the conductive paste is full-cured, interstices between metal particles in the conductive paste are narrowed, and as a result, the metal particles become dense thereby allowing electric current to flow, and thus electric connecting (conductive material) is produced. Metal particles in the conductive material thus obtained, however, are not fused with one another, and are not metal-bonded, either. For this reason, an electric resistivity obtained exceeds 5×10−5 Ω·cm in the case of this method, and to decrease the electric resistivity has been requested, for the purpose of practical application. Further, as to a conductive paste containing an epoxy-type one-component system hardener, a conductive paste containing a hardener having a high curing speed has a problem of lacking stability, while a conductive paste containing a hardener having a good stability has problems of a low curing speed and a poor adhesivity. Further, a conductive paste containing a phenol resin as a binding agent has appropriate adhesivity and stability, but has a problem of poor flexibility. Therefore, such a conductive paste has the following problem: in the case where an article on which the conductive paste is printed is flexible, cracking or delamination occurs in a printed circuit upon folding or bending.
Besides, anther conductive paste that contains, in place of the epoxy resin, an urethane polymer (liquid-form thermosetting resin), a polyhydric alcohol, a petroleum resin having a hydroxy group, and a conductive powder is known (see, for example, Patent Document 1). This conductive paste has characteristics of excellent adhesion to a circuit board, excellent flexibility, and a high curing speed. However, when the urethane polymer is full-cured, interstices between particles of the conductive powder are narrowed and the conductive powder becomes dense, which allows electric current to flow, and therefore a conductive material obtained from such a conductive paste has a relatively high electric resistivity of about 3.0×10−4 Ω·cm to 7.0×10−4 Ω·cm. The conductive powder in the conductive material is not fused, and is not metal-bonded, either.
Further, another conductive resin paste is known which is composed of a resin mixture, a solvent, and a monodisperse fine-particle silver powder, wherein the resin mixture contains a thermosetting resin, a hardener, and a curing accelerator, the thermosetting resin being formed of at least one of an epoxy resin, a phenol resin, and a mixture of an epoxy resin and an imide resin, while the solvent is formed of glycidyl ether and an ester of a lower monoprotic acid and a lower primary or secondary alcohol (see, for example, Patent Document 2). Ester used as a solvent is intended to adjust the viscosity of the paste, to suppress the dissolution with a encapsulant or the like, to improve the storage stability, and to improve the dispersibility of the silver particles. Most of this ester volatilizes when the thermosetting resin in the conductive resin paste is full-cured. However, this conductive resin paste is also a type that allows electric current to flow when interstices between particles of the monodisperse fine-particle silver powder are narrowed thereby making the metal particles denser, and therefore an obtained conductive material has a low conductivity. Further, the silver powder in this conductive resin material is not fused, and is not metal-bonded, either.
Still further, as a conductive adhesive for surface mounting technology, a silver paste composed of a silver filler, an alcohol substance, and an epoxy resin is known, in which the alcohol substance has at least two —OH functional groups (hydroxy groups) per one molecule and is solid at room temperature (see, for example, Patent Document 3). This silver paste is applied over an electrode on a substrate and an electrode of an electronic component is caused to adhere thereto, whereby the electronic component is caused to adhere to the substrate. If this conductive adhesive for surface mounting technology is used, the alcohol substance therein eliminates any oxide film on the electrode of the substrate and any oxide film on the electrode of the electronic component, whereby a point contact is provided easily between the electrode of the substrate and the electrode of the electronic component. Further, a heat treatment to this conductive adhesive for surface mounting technology causes the epoxy resin in the silver paste to be full-cured. However, this conductive adhesive for mounting also allows electric current to flow when interstices between metal particles in the silver filler are narrowed thereby making the metal particles denser, and therefore an obtained conductivity is poor. Still further, the silver filler in the conductive material obtained from this conductive adhesive for surface mounting technology is not fused, and is not metal-bonded, either.